4.3 Connectors, Item Flows & Flow Specifications

4.3 Connectors, Item Flows & Flow Specifications

Quick Summary: On an Internal Block Diagram, parts and ports cannot interact unless they are connected. A connector is a structural line that binds two or more connectable elements within the context of an enclosing block. SysML categorizes connectors into assembly connectors (linking peer parts inside the enclosing block) and delegation connectors (linking outer boundary ports to internal part ports). While connectors define the structural transmission path, item flows specify what actually travels along that path (data, matter, or energy), rendered as a small filled black arrowhead on the connector line.

Structural system decomposition requires rigorous modeling of the communication channels and physical transfer lines that link system components. It is not enough to declare that a system contains a hydraulic pump, a valve, and an actuator; the model must formally specify how the pump feeds the valve, how high-pressure fluid circulates, and how control signals route from the vehicle computer. SysML provides connectors, item flows, and flow specifications to construct an unambiguous structural wiring diagram.

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Connectors in SysML: Structural Interconnection

A connector is a structural feature that specifies a runtime link between connectable elements (such as parts, ports, or reference properties) within an enclosing block:

• Notation: Rendered as a solid line spanning between two elements on an IBD.
• Typing: A connector may optionally be typed by an association defined on a BDD, which provides formal typing for the link.
• Endpoints: Connectors can attach directly to part boundaries (part-to-part), to ports located on part boundaries (port-to-port), or between enclosing block frame ports and internal part ports.

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Assembly Connectors vs. Delegation Connectors

A critical distinction on the OCSMP Model User exam is the classification of connectors based on their architectural tier:

1. Assembly Connectors

• Definition: Connects two internal peer parts, or ports on two internal peer parts, located inside the same enclosing block.
• Context Level: Operates entirely inside the enclosing block at the same hierarchical tier.
• Purpose: Establishes peer-to-peer collaboration, communication, and item exchange between internal subsystems.
• Visual Clue: Both ends of the line terminate on internal parts or ports on internal parts inside the diagram canvas.
• Example: A connector linking the serial output port of gpsReceiver : GPS to the navigation input port of guidanceComputer : FlightComputer.

2. Delegation Connectors

• Definition: Connects an external boundary port on the enclosing block (the diagram frame) to an internal port on a constituent part usage (or vice versa).
• Context Level: Bridges across hierarchical boundaries, connecting the outside world to the internal implementation.
• Purpose: Preserves encapsulation. When an external system sends a signal to the enclosing block's public boundary port, a delegation connector forwards (delegates) that signal inward to the internal part responsible for processing it. Similarly, internally generated outputs are delegated outward to the enclosing block perimeter.
• Visual Clue: One end of the line touches a port located directly on the outer diagram frame (or enclosing block border), and the other end touches a port on an internal part.
• Example: A connector linking the enclosing block's perimeter port ambientAirInlet to the internal part port compressorUnit.inlet.

+-------------------------------------------------------------------------+
| ibd [Block] EnvironmentalControlSystem [Internal Routing]               |
|                                                                         |
|   [perimeterIn]                                                         |
|        |                                                                |
|        | (Delegation Connector)                                         |
|        v                                                                |
|   +----+------------------+            +-----------------------+        |
|   |  [inlet]              |            |  [inlet]              |        |
|   |    compressorUnit     |---(Assy)---|     heatExchanger     |        |
|   |              [outlet] |            |              [outlet] |        |
|   +-----------------+-----+            +-------------------+---+        |
+-------------------------------------------------------------------------+

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Item Flows: Specifying Dynamic Transfer across Connectors

While a connector provides the structural channel or physical wire, it does not inherently state what travels across that channel. In SysML, the dynamic conveyance of matter, energy, or data across a connector is specified using an Item Flow.

Visual Notation of an Item Flow

On an IBD, an item flow is visually represented by:

1. A small filled black arrowhead drawn directly on the connector line.
2. The arrowhead points in the direction that items flow during system operation.
3. A text label placed adjacent to the arrowhead specifying the item flow name and/or item classifier.

+-------------+                                    +-------------+
|   partA     |----------------- ▶ ----------------|    partB    |
+-------------+         telemetryData :            +-------------+
                        TelemetryPacket

Distinguishing the Item Flow Arrowhead from Other Arrows

Exam candidates must be vigilant in distinguishing SysML arrowheads:

• Item Flow Arrowhead: A small filled black arrowhead resting on the connector line pointing in the flow direction.
• Navigability Arrowhead: An open arrowhead (-->) located at the very end of an association line touching a block border.
• Association Name Reading Direction: A small filled triangle (▶) placed floating beside the association text name on a BDD, denoting English reading direction.
• Dependency / Allocation Arrow: A dashed line with an open arrowhead (-.->).

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The Mandatory Item Flow Realization Rule

One of the most frequently tested semantic rules in SysML is the realization dependency of item flows:

Fundamental Rule: An item flow never floats on its own. SysML 1.2 requires "a Connector or an Association" between its source and target, so on an IBD every item flow rides on an underlying connector. (On a BDD, an item flow can be shown on an association.)

If you see two parts on an IBD with an item flow arrow between them, there must be a valid structural connector connecting those parts (or their ports). You cannot draw an item flow floating across empty space between unlinked components.

Item Properties and Flow Payloads

An item flow names the classifier it conveys (a block, value type, or signal). It may also have an optional itemProperty, typed by a block or value type, that records the flowing item as a property of the enclosing block:

• A conveyed «block» represents physical matter or structured data (e.g., HydraulicOil, DataPacket).
• A conveyed «valueType» represents a quantity (e.g., GallonsPerMinute, ElectricCurrent).
• A conveyed «signal» represents an asynchronous message.

The label shows the conveyed classifier (e.g., Torque) or, when there is an item property, name : Type (e.g., torque : Torque). When several item flows share one direction on the same connector, SysML 1.2 draws a single triangle and lists them separated by commas.

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Flow Specifications and Flow Properties on Connectors

When non-atomic flow ports are connected via connectors, their internal flow properties govern the allowable traffic across the connector:

1. Typing Contract: The flow ports at each end of the connector are typed by a «flowSpecification».
2. Conjugation Matching: On an assembly connector between peers, one end is typically regular (FlowSpec) and the opposite end conjugated (~FlowSpec), because SysML 1.2 requires opposite directions (or inout) between external peers. On a delegation connector the rule reverses: the boundary port and the internal port must have the same direction.
3. Realized Item Flows: Each individual flowProperty declared in the flow specification (such as in commandStream, out telemetryFrame) can be realized as a distinct item flow traveling in its respective direction across that single structural connector.

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Comprehensive Comparison Tables

Table 1: Assembly Connectors vs. Delegation Connectors

Table 2: Interconnection Elements: Connectors vs. Item Flows vs. Flow Ports

Table 3: Connector and Item Flow Modeling Rules & Common Pitfalls

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Worked Engineering Scenario: Aircraft Environmental Control System (ECS)

Consider the internal structure of an aircraft Environmental Control System modeled inside an enclosing block:

1. Enclosing Block: EnvironmentalControlSystem

   • Outer frame: ibd [Block] EnvironmentalControlSystem [Internal Thermal Routing]
   • Boundary port on top frame: bleedAirInlet : HotAirStream
   • Boundary port on right frame: cabinConditionedSupply : FreshAirStream

2. Internal Part Usages:

   • preCooler : HeatExchanger [1]
   • compressorUnit : AirCycleMachine [1]
   • mixerUnit : AirDistributionManifold [1]

3. Connector and Item Flow Architecture:

   • Delegation Connector 1: Runs from the frame port bleedAirInlet to preCooler.inletPort. Forwards high-pressure bleed air from the jet engine into the ECS.
   • Assembly Connector 1: Connects preCooler.outletPort to compressorUnit.inletPort.
     • Realized Item Flow: Decorated with a small filled black arrowhead pointing from preCooler to compressor, labeled cooledBleedAir : CompressedAir.
   • Assembly Connector 2: Connects compressorUnit.outletPort to mixerUnit.inletPort.
     • Realized Item Flow: Decorated with a small filled arrowhead pointing into the mixer, labeled chilledAir : ConditionedAir.
   • Delegation Connector 2: Runs from mixerUnit.supplyPort to the frame port cabinConditionedSupply. Routes conditioned air out to the passenger cabin.

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Common Exam Traps & Pitfalls

• Trap 1: Confusing Item Flow Arrowheads with Navigability Arrows: A small filled black arrowhead sitting directly on the line indicates an item flow. An open arrowhead at the terminus of a line indicates navigability. The exam will frequently show an item flow on a connector and ask if it restricts software navigability; it does not.
• Trap 2: Believing an Item Flow Can Replace a Connector: Candidates often incorrectly assume that drawing an item flow makes a connector unnecessary. An item flow is an abstraction that must be realized by a connector.
• Trap 3: Conflating Assembly and Delegation Connectors: If a line connects an internal part to another internal part, it is an assembly connector. If a line touches a port on the outer diagram frame, it is a delegation connector. Reversing these terms is a classic exam distractor.
• Trap 4: Missing Conjugation on Non-Atomic Connectors: When two peer parts connect via non-atomic flow ports typed by the same flow specification, one port must be conjugated (~) so every out meets an in (inout matches either). The rule reverses for a delegation connector: the boundary port and the internal port it delegates to must have the same direction, so neither is conjugated.